Usually, the data recording surface of a write-enabled (reloadable) or rewrite-enabled optical disk is formed from grooves (groove portion) formed in a spiral shape and lands (ridge portion) formed between said grooves. On the wall surfaces of the grooves (the side surfaces of lands), portions of waviness known as wobbles, are formed repeatedly at a fixed period.
Typical physical types of DVDs (Digital Versatile Disk) include DVDs-R (reload type) and DVD-RW (rewrite type). For these types of DVDs, data are recorded on the grooves, and pits (marks) for address information known as land pre-pits (LPP) set according to prescribed rule are formed on the lands.
As shown in FIG. 13, when grooves on an optical disk in DVD-R format or DVD-RW format are tracked by an optical pickup, the returning light of laser beam LB or reflected laser light LB′ is subject to differential detection using the push-pull method, and, as a result, a nearly sinusoidal push-pull signal (SA+SB)−(SC+SD) is obtained corresponding to the waviness shape of the wobbles. Because the push-pull signal contains pyramidal land pre-pit signal SLPP indicating land pre-pits, by applying the push-pull signal (SA+SB)−(SC+SD) on a comparator to be converted to a binary form with a prescribed threshold, one can extract or detect the land pre-pit signal SLPP.
In practice, as shown in FIG. 14, before recording of data on tracks (grooves) (A) and after that (B), there is a significant difference in the waveform of the push-pull signal (SA+SB)−(SC+SD) and in the waveform of land pre-pit signal SLPP. That is, before recording, push-pull signal (SA+SB)−(SC+SD) is maintained in a nearly sinusoidal shape, while land pre-pit signal SLPP has a nice pyramidal protrusion with high amplitude from near the minimum peak value of the wobble signal. Consequently, it is easy to convert land pre-pit signal SLPP to a binary form or to extract the signal. However, after recording, due to the influence of crosstalk by a RF signal component or data component recorded on the same track or on adjacent tracks, the push-pull signal (SA+SB)−(SC+SD) cannot maintain the nearly sinusoidal waveform, and, at the same time, the amplitude of the land pre-pit signal SLPP decreases so that there is little difference in the amplitude from that of the RF signal component, and it is very hard to perform conversion of SLPP to binary form and extraction. Because it is impossible to correctly detect land pre-pit signal SLPP, it is impossible to get the address of the recording data, and many errors take place in reloading, rewrite, or reproduction.